Download DF Df dF df

Gene linkage
In 1909 Bateson, Saunders and Punnett found a cross in sweet peas
that showed a deviation from the law of independent assortment
In this plant, purple flower colour is dominant over red, and
long pollen is dominant over round pollen. True breeding
(BBLL and bbll) flowers were cross and the expected F1
generation was BbLl.
However, when the the F1 generation BbLl was selffertilized, a weird outcome was observed in the F2:
Larger than expected
amount of the P
generation phenotypes
• Some genes are linked and are thus inherited
together
• Linked genes do not assort independently
– Since the linked genes are on the same
chromosome, this makes sense
In the previous example, since the genes
are linked we were more likely to see the
parental (P) type because the genes were
remaining together on the same allele
Look at a cross between DDFF and ddff, what we’ve seen before where
two genes aren’t linked
P generation
DDFF
X
ddff
DDFF can only produce DF gamete
ddff can only produce df gamete
F1 generation
DdFf
DF
Possible gametes?
Df
dF
df
Which gametes are parental?
Which gametes are recombinant
(a mix from the two different
alleles)?
If alleles on the same chromosome are
segregating together during meiosis, how do
we get recombinant (non-parental)
combinations of alleles produced?
A
A
B
B
a
a
b
b
Parents
F1 offspring
X
A
a
B
b
Normally with a double heterozygous
individual, they make 4 possible gametes
Here, only two possible gametes
(parental gametes) since the two
genes are linked.
It is possible to get recombinant gametes (Ab, aB) from crossing
two F1 offspring because we saw non-parental phenotypes in
1909 example (just in a lower than usual quantity). So where are
these Ab and aB combinations coming from in gametes if the
genes are linked?
Check your understanding
Q1) Assume no linkage
Two flies (DDff and ddFF) are crossed producing
DdFf in the F1 generation
What are the possible allele combinations in
the gametes of the F1? State if which are
parental and which are recombinant.
PDf, dF
RDF, df
Q2) Normally, the F2 has a phenotypic ratio of 9:3:3:1 if unlinked (9,
1 being parental combinations). If the two genes are linked, how
would the proportion of parental combinations be affected?
They would both increase (since crossing over is relatively rare)
Q3
Consider the following cross results:
Normally:
9/16=56.25%
3/16=18.75%
3/16=18.75%
1/16=6.25%
Total: 556
Are seed colour and shape linked? Why or why not?